In implementing fully integrated wireless transmitter systems, CMOS RF power amplifiers have been an important component block to handle RF signals carrying modulated data. To support high data-rate wireless transmission, the power amplifiers should be able to deliver very high output power to antennas while maintaining good power efficiency. Most importantly, the linearity of the power amplifiers must be good enough not to distort modulated data signals.
A power combining technique is a good means to generate high output power out of moderate output powers of individual power amplifiers. Because the required output power level for individual amplifiers can be lowered as the number of combined amplifiers increases, individual amplifiers can have more linearity margins. If the RF signal swing at the input node of transistor in the output power stage can be kept low, there will be less signal distortions by non-linear function of saturated output transistors. Discrete power controllability of multi-combined power amplifier is another advantage in terms of enhancing the power efficiency. In typical power amplifiers, overall efficiency, which is the ratio of transferred output power to DC power dissipation, drops radically as the output power level is lowered. If the number of functioning PAs is reduced by disabling part of active amplifying paths as the required output power level is lowered, overall power efficiency can be enhanced by saving DC power dissipation of the inactive PAs.
Conventional power combining techniques use multiple power transferring paths that are combined by output matching networks such as transformers. For discrete control of PA systems to improve efficiency at power back-offs, those conventional structures activate or inactivate individual power transferring paths by turning-off unnecessary PAs, so that inactive PAs are not contributing to the power combining at the output matching network (transformer in typical case). Because negative effects of idle PAs on output matching networks are negated in the conventional structures, the maximum available efficiency of the output matching networks cannot be acquired.